1. Field of the Invention
The present invention relates to a piezoelectric bulk wave filter known as a bandpass filter, for example. More specifically, the present invention relates to a longitudinally-coupled multi-mode piezoelectric bulk wave filter, and an electronic component including such a piezoelectric bulk wave filter using vibrations of different orders based on the piezoelectric longitudinal effect.
2. Description of the Related Art
A variety of piezoelectric bulk wave filters are used as band filters. Dual-mode piezoelectric bulk wave filters are mainly used within a frequency range of several MHz to tens of MHz, because of the ease of miniaturization and low cost.
A dual-mode piezoelectric bulk wave filter is disclosed in Japanese Unexamined Patent Application Publication No. 5-327401.
FIG. 20 is a cross-sectional view of a conventional dual-mode piezoelectric bulk wave filter using a thickness longitudinal vibration.
A piezoelectric bulk wave filter 201 includes a piezoelectric substrate 202 that is polarized in the direction of thickness. A pair of excitation electrodes 203 and 204 is disposed on the top surface of the piezoelectric substrate 202. A common excitation electrode 205 is opposed to the excitation electrodes 203 and 204 with the piezoelectric substrate 202 sandwiched therebetween.
During use, an input signal is applied between one excitation electrode 203 and the common excitation electrode 205 to excite the piezoelectric substrate 202. When the piezoelectric substrate 202 is excited, a symmetrical mode shown in FIG. 21A and an anti-symmetrical mode shown in FIG. 21B are generated. The two modes are coupled, forming a filter bandwidth. An output is picked up between the excitation electrode 204 and the ground electrode 205.
Also known in addition to the dual-mode piezoelectric bulk wave filter using the thickness longitudinal mode is another dual-mode piezoelectric bulk wave filter which includes the piezoelectric substrate 202, polarized in a direction parallel to the top surface, and utilizing a shear vibration mode.
The degree of coupling between the symmetrical mode and anti-symmetrical mode in the conventional piezoelectric bulk wave filter 201 depends on the spacing between the excitation electrodes 203 and 204. The spacing determines a frequency difference between the symmetrical mode and anti-symmetrical mode, thereby determining a passband.
Specifically, to produce a wide band filter, the spacing between the excitation electrodes 203 and 204 must be narrowed to increase the degree of coupling between the two modes, and to increase the frequency between the two modes.
The excitation electrodes 203 and 204 are typically produced using a method of screen printing electrically conductive paste. The screen printing technique is subject to limitations in the ability to narrow the spacing between the excitation electrodes 203 and 204. If the excitation electrodes 203 and 204 are produced using a photolithographic technique, the spacing between the excitation electrodes 203 and 204 is narrowed, but the costs involved increase.
Even if the spacing between the excitation electrodes 203 and 204 is narrowed, a capacitance between the input and output increases in the piezoelectric bulk wave filter 201, leading to a smaller attenuation.